What Is Static Friction?

Static friction is when an object has a relative movement tendency on the surface of another object, but when the relative movement does not occur, the force that hinders the relative movement tendency of the object.

Chinese name: Static friction
Foreign name: static friction

Subjects: Physics-mechanics
Production conditions: Contact surface is rough

Basic definition of static friction

When an object has a relative motion trend on the surface of another object, the force that hinders the relative motion trend of the object is called static friction. Three points should be noted here: one is that two objects are in close contact and are relatively stationary; the other is that they have a tendency to slide or roll relatively, but no relative sliding or rolling has occurred. Three is that two objects must be rough. Objects also have static friction on inclined surfaces.

Conditions for generating static friction

The contact surface is rough;

Two objects are in contact with each other and there is a force between them;

There is a tendency of relative movement between objects;

The two objects are relatively still.

Direction of static friction

Direction: The direction of static friction is always along the contact surface, but when the contact surface is curved, it is tangent to the contact surface and opposite to the direction of the relative movement of the object. The so-called relative is based on a friction force-applying object.

Judgment of the direction of the relative movement trend: Assuming that the contact surface is smooth without friction, look at the relative movement direction of the object, and determine the direction of the relative movement trend.

Magnitude of static friction

See Figure 2. The device gradually moves horizontally, so the static friction between the block and the tabletop no longer exists. According to the analysis above:

Maximum static friction: There is a maximum static friction, which is called maximum static friction. It is equal to the minimum external force required to make the object just move.

The magnitude of static friction is not a fixed value. The static friction varies with the actual situation. The magnitude is between zero and the maximum static friction Fm. Its value can be determined by the motion state of the object. The maximum static friction force is proportional to the contact surface pressure. The maximum static friction force increases with the increase of pressure and decreases with the decrease of pressure.

Static friction is very common. For example, when people walk, they are subject to forward static friction (the heel of a person has a backward tendency relative to the ground. As soon as the heel is off the ground, there is no static friction if there is no power), such as the wheel of a car, rolling friction is essentially static friction) The magnitude of the static friction force can only be solved according to the state of motion (balance or acceleration) of the object, and is solved by the equilibrium conditions and Newton's law, and has nothing to do with the positive pressure (different from the maximum static friction force). For example, bottles and brushes held in your hand will not fall off, which is the result of static friction. The ability to weave threads into cloth and sew the cloth into clothes is also dependent on the static friction between the yarns. There are also many applications of static friction in production technology. For example, belt conveyors rely on the static friction between the cargo and the conveyor belt to move the cargo elsewhere. In addition, the friction between the driving wheels of bicycles and automobiles and the ground also belongs to static friction, that is, the rolling friction also belongs to static friction.

Static friction

When the external force increases to the moment when the object starts to move, the static friction force increases to the maximum value, which is called the maximum static friction force. The maximum static friction is related to the magnitude of the positive pressure, and its value is slightly larger than the sliding friction, but sometimes people think that the maximum static friction is equal to the tactical magnitude of the sliding friction for the sake of calculation convenience.

Forces with the same magnitude and opposite direction of the pulling force F (forces with a tendency to move) are called static friction forces.

Definition: Two objects that are in contact with each other and are relatively still, if there is only a relative sliding tendency under the action of external forces, but no relative sliding occurs, the force that appears between their contact surfaces to prevent the relative sliding is called " Static friction. "

Size: The static friction force increases with the increase of the tangential outward force, but the increase of the static friction force can only reach a certain maximum value. The static friction force f is always equal to the external force on the object along the tangential direction of the contact surface before reaching the maximum value.

Direction: It always reverses the direction of the relative sliding trend before reaching the maximum

Static friction analysis method

Conditional method

Judging by the conditions under which the static friction force is generated. This is the most direct and basic method for analyzing static friction.

Example 1. As shown in Figure 1, there is a triangular wooden block on the rough horizontal plane, and two small wooden blocks of mass m1 and m2 are placed on its two rough inclined planes, m1> m2. Knowing that the triangular wooden block and the two small wooden blocks are stationary, try to analyze the frictional force of the rough horizontally facing triangular wooden block.

Analyze the triangular wooden block and the two small wooden blocks are stationary.

figure 1

figure 2 The three as a whole, as shown in Figure 2. The whole is subject to the support of gravity and the horizontal plane in the vertical direction, and the resultant force is zero; there is no other force in the horizontal direction, and there is no tendency to move with respect to the horizontal plane. Therefore, the rough horizontal surface has no static friction against the triangular wooden block.

2. Hypothesis

Assuming that there is no static friction, analyze the relative motion of the object to determine the effect of the static friction.

Example 2. As shown in Figure 3, the rod AB rests statically on a right-angle wall. The vertical part of the wall is smooth and the horizontal part is rough. Try to analyze the static friction force on the rod.

The analysis assumes that the rod is not subjected to static friction, so the B end of the rod will slide to the right, which indicates that the B end of the rod is subject to horizontal left friction, which hinders the tendency of the rod to relatively face the right.

image 3 Balance method

Analyze based on the condition that the object is in equilibrium.

(1) Balance condition using common point force: Fhe = 0

Example 3. As shown in Figure 4, a mass of mass m is placed on a fixed slope with an inclination angle , and the dynamic friction coefficient between the mass and the slope is: The friction force f between the block and the inclined plane.

The analysis is shown in Fig. 5. The block is affected by the gravity G, the support force N of the inclined plane, and the external force F. If there is friction between the block and the inclined surface, it is static friction. To determine the movement trend of the block relative to the inclined plane, the magnitude relationship between the external force F and the gravity component mgsin along the inclined plane should be determined first. Since the magnitude of F is unknown, the conditions for equilibrium using common points are discussed as follows:

If F = mgsin, there is no relative movement trend between the block and the inclined plane, so there is no friction between the block and the inclined plane.

If F> mgsin, the block has an upward movement trend relative to the inclined plane. The direction of the static friction force of the inclined plane is downward along the inclined plane, and F = mgsin + f, that is,

f = Fmgsin

If F <mgsin, the block has a downward movement trend relative to the inclined plane, and the direction of the static friction force of the inclined plane is upward along the inclined plane, and F + f = mgsin, that is,

f = mgsin-F.

(2) Balance condition using torque: M-up = 0

Example 4. As shown in Figure 6, an inextensible light rope suspends a uniform, ball of weight G at point A on a vertical wall, the ball is stationary, and the rope is tied to point C on the ball. Try to analyze the direction of the static friction between the ball and the vertical wall.

The force of the analysis ball is shown in Figure 7. With the center of the sphere as the fulcrum, the resulting moment is zero. Among them, the gravity G and the supporting force N pass through the center of the sphere, and the moment is zero. Therefore, the moment of the static friction force is equal to the moment of the rope pulling force and the direction is opposite. Therefore, the direction of the static friction force at B must be vertically upward, resulting in a counterclockwise The moment is balanced with the clockwise moment produced by the pulling force T of the rope.

4. Analysis using Newton's second law

Example 5. As shown in Figure 8, two objects A and B are stacked on a fixed smooth inclined surface and slide together to find the static friction force on B.

The analysis first takes A and B as a whole as the research object. Because the slopes are smooth, the acceleration of A and B sliding together is a = gsin.

Taking object B as the research object, its acceleration is also a. As shown in Figure 9, a is decomposed into ax and ay in the horizontal and vertical directions. Analyzing the force of B, it can be known from Newton's second law that the combined force of gravity and supporting force generates acceleration ay in the vertical direction, and the acceleration ax in the horizontal direction should be generated by the horizontal static friction force to the left, so

Direction to the left.

5. Analysis using Newton's third law

Example 6. As shown in Figure 10, under the action of force F, both A and B objects with mass m are stationary. Try to analyze the static friction force that A receives.

Analysis directly isolate A is difficult to analyze its stress situation. B can be isolated first, as shown in Figure 11, it can be seen that the static friction between A and B and the gravity of B are a pair of equilibrium forces, that is, the static friction between A and B fAB = mg, and the direction is vertically upward. According to Newton's third law, the static friction force of B against A is fBA = mg, and the direction is vertical downward. Since A is in a balanced state, it can be known from the equilibrium conditions that A is also subjected to the static friction force of the wall against it, F = 2mg, and the direction is vertical. up.

Summary There are many methods for analyzing the static friction force, and it is necessary to choose an appropriate method according to the characteristics of specific problems. The application of the above methods is often not independent. Pay attention to the connection and organic combination.